When it is sought to increase the density of information written on an optical disk, this is generally limited by the performance of the device for reading the information. The basic principle is that physical information written on the disk can only be read with great difficulty if its dimensions are less than the resolution limit of the optical system which will be used for reading that information. Typically, when reading with a red laser of wavelength 650 nm and a numerical aperture of 0.6, it is not normally possible to hope to correctly read information having dimensions less than 0.4 micrometer, or at the least 0.3 micrometer. With a blue laser of wavelength 400 nm, it will not be possible to read marks having dimensions less than 0.2 or 0.3 micrometers.
However, method called super-resolution methods have been envisaged for reading information whose physical dimensions are smaller, or even very much smaller, than the wavelength. These methods are based on the non-linear optical properties of certain materials. Non-linear properties is understood to mean the fact that certain optical properties of the material change as a function of the intensity of the light which they receive. The direct causer of this change can be the thermal heating due to this illumination: it is the reading laser itself which will locally modify the optical properties of the material by thermal, optical, thermo-optical and/or optoelectronic effects on dimensions smaller than the dimension of the reading laser spot; because of the change of property, an item of optical information present in this very small volume becomes detectable whereas it would not have been detectable without this change.
The phenomenon used is principally based on two properties of the reading laser that will be used:                on the one hand, the laser is focused very strongly in order to exhibit an extremely small cross-section (of the order of the wavelength) but whose power distribution is Gaussian, very strong at its center and very attenuated at the periphery,        and, on the other hand, a reading laser power is chosen such that the power density over a small part of the cross-section, at the center of the beam, significantly modifies an optical property of the layer, whilst the power density outside of this small portion of the cross-section does not significantly modify this optical property; the optical property is modified in a way tending to allow the reading of an item of information which would not be readable without this modification.        
For example, the optical property which changes is an increase in the optical transmission in the case where the reading of a bit constituted by a physical mark formed on the optical disk necessitates a transmission of the laser beam up to this physical mark. The non-linear layer is then interposed in the path of the beam towards the physical mark. The center of the laser beam will be able to pass through the layer up to the mark, because on passing through the layer the intensity of the incident light will make it more transparent, whilst the periphery of the beam will not pass through because it does not sufficiently modify the optical indices of the layer in order to make it more transparent. Everything happens as if a beam focused onto a much smaller diameter than would be allowed by its wavelength had been used.
Various theoretical proposals have been formulated in order to implement these principles, but none of them has given rise to industrial development. The patent U.S. Pat. No. 5,153,873 reviews the theory. In the patent EP 1492 101 there is described a super-resolution layer constituted of nanoparticles of metal, accompanied by a specific stack containing a phase changing material.